Use DL preferred alignment for alloca in Value::getPointerAlignment

Use DL preferred alignment for alloca in Value::getPointerAlignment

Teach Value::getPointerAlignment that allocas with no explicit alignment are aligned to preferred alignment of the allocated type.

Reviewed By: hfinkel

Differential Revision: http://reviews.llvm.org/D17569

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@267689 91177308-0d34-0410-b5e6-96231b3b80d8